ABSTRACT TRIBOLOGICAL PROPERTIES OF A TIGHTLY WOVEN CARBON/CARBON COMPOSITE Karaveli, Keriman M.Sc., Department of Metallurgical and Materials Engineering Supervisor: Prof. Dr. Abdullah &Ouml / zt&uuml / rk June 2005, 77 pages Tribological properties of a tightly woven Carbon/Carbon (C/C) composite were assessed experimentally in accord with the ASTM pin on disk technique. The C/C composite used in this study was a commercial material (K-Karb) obtained in a panel form. The composite consists of graphite fiber reinforced graphite matrix developed for aerospace applications. The fiber reinforcement was in a plain weave woven fabric form. The tests were conducted by sliding zirconia ball against the C/C composite. The friction coefficient and wear rate were determined as functions of applied load, sliding speed, sliding distance and lubrication in ambient laboratory conditions. Mean friction coefficient of the composite was 0.135 &micro / when tested at ambient atmosphere and 0.113 &micro / in lubricated environment at a load of 5 N, sliding speed of 0.5 cm/s, and sliding distance of 100 m. The wear volumes determined from surface profile traces obtained on the wear tracks after completion of the tests were used for calculations of the specific wear rates. The specific wear rates of the composite were 0.754 x 10-4 mm3/N.m at ambient atmosphere and 0.437 x 10-4 mm3/N.m in lubricated environment at the load of 5 N, sliding speed of 0.5 cm/s, and sliding distance of 100 m. The specific wear rate of the composite decreased with increasing sliding distance, sliding speed, applied load and also, decreased in lubricated environment. Keywords: C/C composite, tribology, friction, wear, lubricant.

Un procédé de synthèse de dépôts nanostructurés de céramiques à bases d’aluminium au sein de composites carbone/carbone (C/C) est développé. Il consiste à synthétiser à partir de précurseurs dissout en solution aqueuse des nano-particules de morphologies variées. La maîtrise du procédé se déroule en deux étapes. Dans un premier temps une étude sur substrat plan permet de saisir les points clés du procédé, puis l’adaptation de la synthèse au sein d’un échantillon massif permet le changement d’échelle à des échantillons fibreux. Quatre gammes d’éprouvettes de matériaux densifiés sont ainsi élaborées. La caractérisation des matériaux permet de prouver la bonne cohésion de ceux-ci et de mesurer plusieurs propriétés mécaniques et structurales.En complément plusieurs interfaces carbone|alumine ou nitrure d’aluminium sont simulées par une approche de dynamique moléculaire ab initio. La méthode choisie pour générer les modèles consiste à simuler la trempe d’un carbone amorphe à haute température sur une surface céramique immobile, puis de relaxer les contraintes.Des disparités sur l’organisation structurale des nano-structures et en particulier sur l’orientation des plans de graphène générés vis-à-vis de la surface sont observées en fonction des modèles. Lorsque les plans sont plutôt perpendiculaires à la surface, l’interface est constituée de nombreuses liaisons fortes et le comportement en simulation de traction est bon, tandis que des plans parallèles entrainent une interface et un comportement faible.Enfin, la caractérisation HRTEM de certains matériaux élaborés permet d’identifier expérimentalement des interfaces semblables à celles obtenus par les modèles. / A hydrothermal-like process to introduce nano-structured alumina and aluminum nitride in carbon/carbon (C/C) composites is developped. Starting from dissolved reactants in an aqueous media,nanoparticules with various morphology are synthesised. The understanding and control of the processis completed in two steps. In the former the study on simplewafer type substrat allows to identifythe key parameters of the process. During the latter a scaling up of the process is done to allow thesynthesis in situ of C/C composites. The microstructural and some mechanical characterization ofthe four ranges of material produced is achieved.In addition several carbon|alumina and carbon|aluminumnitride are simulated using an ab initiomolecular dynamic approach. The methdology to generate the models consists in sumulating theliquid quench of a high temperature amorphous carbon inbetween fixed ceramic surface, then torelease the constrains. Depending on the system, different organisations of the nano-carbons withinthe surfaces are identified : when the graphene sheets are pependicular to the surface, the modelshows an important number of strong bonds and the simulation traction behavior is good, whereaswhen they are parallel to the surface it leads to weak interface and mechanical behavior.Finally HRMET charasterization of some of the materials produced allows to identify experimentalinterfaces alike to those obtained during themolecular dynamic simulations.

In recent times, composite materials have gained mainstay acceptance as a structural material of choice due to their tailorability and improved thermal, specific strength/stiffness and durability performance. Carbon-Carbon (C/C) composites are used for high temperature applications such as exit nozzles for rockets, leading edges for missiles, nose cones, brake pads etc. Mechanical property estimation of C/C composites is challenging due to their highly heterogeneous microstructure. Computed Tomography (CT) images (volumetric imaging) coupled with Scanning Electron Microscopy (SEM) reveal a highly heterogeneous microstructure comprised of woven C-fibers, amorphous C-matrix, irregularly shaped voids, cracks and other inclusions. The images also disclose structural hierarchy of the C/C composite at different length scales. Predicting the mechanical behavior of such complex hierarchical materials like C/C composites forms the motivation for the present work. A systematic study to predict the effective mechanical properties of C/C Composite using numerical homogenization has been undertaken in this work. The Micro-Meso-Macro (MMM) principle of ensemble averages for estimating the effective properties of the composite has been adopted. The hierarchical length scales in C/C composites has been identified as micro (single fiber with matrix), meso (fabric) and macro (laminate). Numerical homogenization along with periodic boundary conditions (PBCs) have been used to estimate the effective engineering properties of the material at different length scales. Concurrently, mechanical testing has also been carried out at macro (compression tests) and micro scale (using nano-indentation studies) to characterize the mechanical behavior of C/C composites.

Les composites Carbone/Carbone (C/C), largement utilisés à hautes températures dans des environnements oxydants, sont sensibles à l'oxydation dès 400°C. Deux voies sont envisagées pour accroître la tenue à l'oxydation de ces matériaux. La première est de renforcer la force de la liaison fibre/matrice en modifiant les propriétés de surface des fibres de carbone. La seconde voie d 'étude consiste à remplacer une partie de la matrice carbone par un oxyde. L'introduction d'une nouvelle phase conduit à la création de nouvelles interfaces dans les matériaux. L'ensemble des résultats obtenus à partir des caractérisations physico-chimiques et macroscopiques des matériaux élaborés permettent alors de répondre à la problématique qui est de connaître le rôle des interfaces dans les propriétés macroscopiques de composites C/C. / C/C composites are widely used as structural parts in oxiding environment. Yet, they become very sensitive to air oxidation for temperature higher than 400°C. This work proposed two methods to improve their oxidation resistance :(i) the fiber/matrix bond is increased by modifying the properties of the carbon fiber surface, and (ii) a fraction of the carbon matrix is substitued by an oxide phase.Introduction of new constituants leads to the creation of new material interfaces. Thus, the influence of the interfaces on the macroscopic properties of the composite is evaluated by physico-chemical and macroscopic characterizations.